THE RESPONSE OF HUMPBACK WHALES (MEGAPTERA NOVAEANGLIAE) TO OFFSHORE SEISMIC SURVEY NOISE: PRELIMINARY RESULTS OF OBSERVATIONS ABOUT A WORKING SEISMIC VESSEL AND EXPERIMENTAL EXPOSURES

Abstract

During October to November 1996 WMC Petroleum conducted the Robert 3D seismic survey to the northeast of North West Cape, off Exmouth Western Australia. The survey used one of two 2678 cui (44 L) air-gun arrays of source level 258 dB re 1µPa2−m p−p below the array (77 bar-m), operating every eight s for 33.4 days of continual operation. Tracklines ran east-west across the path of southerly migrating humpback whales. Before the seismic survey began aerial surveys determined that humpbacks moving through the seismic area were distributed uniformly seaward of the 20 m depth contour. Based on observations made from the operating Geco Resolution there did not appear to be any gross changes in the migratory path of humpback whales through the seismic area. Whales approaching the operating Geco Resolution began avoidance manoeuvres at 5−8 km and mostly kept a standoff range of 3−4 km. Some whales approached the vessel closer. Experiments were carried out in Exmouth Gulf where the movements and behaviour of humpback pods were monitored before, during and after an approach with a 20 cui air-gun (0.33 L) of horizontal source level 227 dB re 1µPa2−m p−p. The levels at which avoidance manoeuvres began during these trials was approximately 159 dB re 1µPa2 p−p, which is roughly equivalent to the received level of the 2678 cui array at 5 km, at 162 dB re 1µPa2 p−p. General avoidance of the 20 cui air-gun was observed at 1 km, or a level of 168 dB re 1µPa2 p−p, which was roughly equivalent to the level of the 2678 cui array at 3 km at 170 dB re 1µPa2 p−p, which was the general minimum humpback standoff range observed from this array. Whales were observed to move closer to the operating 20 cui and 2678 cui air-gun/array. It is speculated that these whales were mostly males intent on investigating or passing quickly by the appropriate air-gun/array.